docs: def _transform_col(self, x, i):
          """Encode one categorical column into labels.
  
          Args:
              x (pandas.Series): a categorical column to encode
              i (int): column index
  
          Returns:
              x (pandas.Series): a column with labels.
          """
          return x.fillna(NAN_INT).map(self.label_encoders[i]).fillna(0)